[tabris]

I'm having a difficult time with the two questions.

Reaction (CH3)3BR + N3- -> (CH3)3CN3

(CH3)3cbR + Br- -> (CH3)3CN3 + Br-
                       (k-1)

(CH3)3cbR + Br- <- (CH3)3CN3 + Br-
                       (k1)

(CH3)3C+ + N3- -> (CH3)3CN3
                       (k2)

a) Derive the rate law for the reaction starting from an expression for the rate of consumption of N3-

What I did so far...

Production of (CH3)3C+ = Consumption of (CH3)3C+
K1[(CH3)3CBr] = K-1[(CH3)3CN3][Br-] + K2[(CH3)3C+][N3]

I wondered if (CH3)3C+ being consumed from K-1 should be equal to (CH3)3C+ consumed in K2, or if adding both K-1 and K2 would reflect 100% of the (CH3)3C+ consumption. I decided that they should be added, though I wasn't 100% sure on it.

Isolate for [N3-]

[N3-] = ( K1[(CH3)3CBr] - K-1[(CH3)3CN3][Br-] ) / (K2[(CH3)3C+] )

b) a) Derive the rate law for the reaction starting from an expression for the rate of consumption of (CH3)3CBr

So... same idea as a). Isolate for [(CH3)3CBr]

[(CH3)3CBr] = ( K-1[(CH3)3CN3][Br-] + K2[(CH3)3C+][N3] ) / (K1)


c) Briefly comment on whether the result from a and b are consistent with each other.

This is where I begin to question if I was on the right track with a and b. Obviously, (CH3)3CBr and N3- should have the same consumption rate since they have same mole ratio from the original equation, and even the split mechanism also shows that the specific amount of (CH3)3C+ produced from (CH3)3CBr (hence, the limiting reagent, in a way?) will have to react to similar amount of N3- (ex. If first mechanism can produce 100 mol of (CH3)3C+, then no matter how much N3- you have, you can only use 100 mol at the time) So this seems to justify the fact that the consumption rate is similar.

But it's not so obvious just by looking at the answer a) and b)... did I miss something here? Ummm... help? I really tried... and I don't know where to go from here.

Another one involves equilibrium

[AuCl4]-(aq) + 3V3+ (aq) + 3H20(l) <-> Au(s) + 4Cl-(aq) + 3VO2+ (aq) + 6H+(aq)

K = 5.0 x 10^33 at 298K

In an experiment, 1 mol KAuCl4 and 1.0 mol V(ClO4)3 are dissolved in water at 298K to make 1L of solution. Calculate the equilibrium concentrations of all the ionic species. Justify any approximations you make. 10^10 (Hint: Becasue Kc >>1, assume the reaction goes 100% to completion and then reserves a little to establish a true equilibrium mixture)

I can set up the simple equilibrium equation, but I'm confused about the hint. As well, the equation I set up makes little sense.

[AuCl4]-(aq) + 3V3+(aq) <-> 4Cl-(aq) + 3VO2+ (aq) + 6H+(aq)
I      1mol/L      1mol/L              0             0                 0
C      -x             -3x               4x           3x               6x
E       1-x            1-3x             4x           3x               6x

(4x)(3x)(6x) / (1-x)(1-3x) = 5.0 x 10^33

No freaken way as to how first year chem course expects this complicated math. As a matter of fact, I'm convinced it's impossible unless you're hardcore student in math department. So I must have done something wrong here. But again, I'm lost.


You'd really make my day if you could help me out with reasons I can understand and accept. I should not have taken this course online...